Draft EIS_072312.pdf - Middle Fork American River Project ...

20120723-4002 FERC PDF (Unofficial) 07/23/2012
space to minimize potential spills from the reservoirs during the next filling period if the
runoff is high (wet year); and (2) retaining enough water in storage to ensure that license
requirements and consumptive demands can be met in the following year if the next
filling period runoff is low (dry year).
Project operations are prioritized to first ensure consumptive water demand
(deliveries) are met and second to maximize peak power generation. However, in all but
dry years, water supply demands are met as a byproduct of power generation because
both consumptive water and electrical demands tend to coincide seasonally. In addition,
the project generally controls and releases far more water annually in most water years
(except in dry years) than PCWA requires to meet the consumptive water demand.
The French Meadows powerhouse generates electricity when water is moved from
French Meadows reservoir to Hell Hole reservoir. It is nearly always operated in blockloaded
conditions with the duration of the block of operation set depending on the
volume of water to be moved.
The Hell Hole powerhouse generates opportunistically from flow releases from
Hell Hole dam. Project operations are not modified for power generation at Hell Hole
powerhouse.
The Middle Fork and Ralston powerhouses generally run in tandem, using water
transported from Hell Hole reservoir to Ralston afterbay. The two powerhouses produce
about 90 percent of the project’s annual generation. Occasionally, PCWA curtails the
operations at the Middle Fork powerhouse to take advantage of accretion flows into the
Middle Fork interbay. Although the Middle Fork interbay is located between the two
powerhouses, the interbay has little ability to re-regulate flows because of its small
storage capacity. If the flows through the Middle Fork and Ralston powerhouses are not
matched, the Middle Fork interbay would either be drained or overtopped very quickly.
PCWA varies releases on a daily and hourly basis through the Middle Fork and
Ralston powerhouses in response to changing electrical demands, grid needs, and water
supply conditions. The powerhouses are often used to help maintain reliable operations
of the transmission grid by fine-tuning the flow of electricity in the grid to balance supply
and demand. When operated to provide grid regulation, flow rates through the Middle
Fork and Ralston powerhouses vary quickly to meet constantly changing energy supply
and demand conditions. The powerhouses are also usually set at an efficient operating
level and run for a prescribed number of hours per day depending upon hydrology.
The Oxbow powerhouse frequently runs in tandem with the Middle Fork and
Ralston powerhouses. The discharge capacity of the Oxbow powerhouse is slightly
higher than the Ralston powerhouse, which allows the Oxbow powerhouse to use water
supplied by the Ralston powerhouse as well as inflow from the Middle Fork American
and Rubicon Rivers. The Ralston afterbay also has sufficient operational storage
capacity to allow the Oxbow powerhouse to operate independently of Middle Fork and
Ralston powerhouses for several hours at a time, depending on generation level. This
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